Method and apparatus for roasting ores



July 6, 1937. c. F. SILSBY 2,086,193

METHOD AND APPARATUS FOR ROASTING ORES Filed March- 15, 1934 INVENTOR61/. Si/shy BY Z 2 Z ATTORNEY Patented July 6, 1937 UNITED STATES METHODAND RPIUS FOR ROASTING Charles Forbes Silsby, White Plains, N. 1., as-

signor to General Chemical Company, New York, N. Y., a corporation ofNew York Application March 15, 1934, SerialNo. 715.628

8 Claims.

This invention is directed to methods and apparatus for roastingmetallic sulfides, and more particularly for roasting finely dividedsulfide ores to desulfurize the .same and to produce sulfur dioxide foruse in the manufacture of sulfuric acid, or for any other purposedesired.

Generally speaking, to a large extent present practice in sulfide finesroasting includes use of mechanically operated multiple hearth furnaces,such, for example, as the well known Mac- Dougal, Herreshofi, and Wedgeburners. While burners of this type provide effective roasting, thecomplicated construction and operation thereof involve considerableinitial and maintenance expense.

As distinguished from the bed roasting operation of these burners, ithas been previously suggested to roast the fines while in gaseous sus-Pension, in operations in which the fines are either injected into aroasting chamber in suspension in the oxidizing gas, or are showereddownwardly into the roasting chamber wherein the fines enter cross orcountercurrents of suspending gas.

Suspension roasting is best applicable when the fines are in arelatively finely divided state, and this method presents the notableadvantage over mechanical multiple hearth operation of considerablylowering costs of production, by reason of elimination of relativelycomplicated and moving parts, expensive rabbling and other apparatusinherent in the construction and operation of such mechanical burners.The present invention is directed especially to improvements insuspension roasting methods.

The principal object of the invention resides in the provision of aprocess which may be carried out so as to provide for the presence ofrelatively large amounts of oxygen in the roasting atmosphere at thebeginning and at the end of roasting of the ore particles, to facilitaterapid ignition of the ore on introduction into the com-.

bustion zone, and to efiect substantially complete oxidation of the oreparticles at the end of the roasting operation.

In carrying out one preferred embodiment of the process of theinvention, the sulfide fines are initially dried and preheated byutilization of heat of the top of the roasting furnace so thatrelatively large quantities of sulfide fines may be dried and preheatedto thus promote rapid ignition of the ore particles on introduction intothe roasting zone. The dried and preheated fines are run into feedmechanisms positioned in the side walls of the furnace at approximatelythe midpoint of the combustion zone. These feed mechanisms inject thefines into the roasting chamber at a relatively high angle so as tocause the particles to rise substantially to the top of the combustionzone, at which point a dispersion of partially roasted fines is formedrelatively uniformly over at least a major portion of the area of thetop of the roasting chamber.

The oxidizing gas, such as air, for supporting roasting of the ore isintroduced into the comtion as a whole is introduced into the combustionzone at the top, preferably under slight plus pressure, there is thusprovided at the upper end of the reaction chamber a zone relatively richin oxygen. The presence of a comparatively large amount of oxygen inthis part of the reaction zone promotes rapid ignition and initialroasting of the ore particles. While dropping through the upper portionof the'reaction zone, above the combustion gas ofitake, the oreparticles pass downwardly, cocurrent with the gases, through anatmosphere decreasingly rich in oxygen. After having passed through theupper section of the combustion zone, the sulfur content of the oreparticles has become depleted, and thereafter roastingv proceeds lessreadily. The ore particles then enter the lower portion of the reactionzone, below the reaction gas ofltake, and pass downwardly countercurrentto a rising stream of oxidizing gas. Thus, during the final stage of theroasting operation the ore particles pass through an atmosphereincreasingly rich in oxygen and decreasingly rich in sulfur dioxide. Inthe lower end of the lower portion of the reaction chamber, conditionsare such that the gas therein is substantially all air and containslittle sulfur dioxide. In this zone, the cinder particles are surroundedby an atmosphere rich in oxygen which promotes and hastens completion ofroasting of the ore particles. The oxide cinder is then discharged fromthe bottom of the roasting zone.

Further objects and advantages of the invention will be apparent from aconsideration of the following description taken in connection with theaccompanying drawing, in which Fig. 1 is a vertical section of apreferred furnace for carrying out the process of the invention Fig. 2is an enlarged vertical section of a preferred type of ore feedmechanism, and

Fig. 3 is a vertical section of the lower end of a modified burner.

Referring particularly to Fig. l of the drawing, l0 designates a shaftburner comprising a shell ll constructed of suitable refractorymaterial, such as firebriclr, and defining a substantially unobstructedroasting chamber i2 of cylindrical cross-section. Surrounding shell IIis a steel casing l3 acting as a protective reinforcement for thefurnace. The upper end of the combustion chamber is closed off by acrown I5, the top side of which forms a drying and preheating hearth l1.The cylindrical shell II and casing l3 project upwardly beyond the crownl5, and carry a steel framework l8 which in turn supports ore feedingand rabble mechanisms for the drying hearth.

The surface of hearth I! is slightly coneshaped and slopes downwardlytoward the shell of the burner. Positioned above the hearth are rabblearms I! having downwardly projecting plows 20 pitched to work sulfidefines gradually toward the circumference of the drying hearth. Arms 9are rotated by a motor 22 through shaft 23 supported in bearings so asto maintain the lower ends of the plows 29 properly spaced with respectto the surface of the drying hearth II. An ore bin 24, mounted onframework l8, discharges ore onto a platform 25 from which fines areintermittently dropped onto approximately the center of hearth II by asweep 2'! rotating with shaft 23.

Cut in shell ll near the upper end are downwardly sloping passages orconduits 94 through which ore is passed from the drying hearth I I ontothe surfaces of screens 35 covering funnels 31 forming the upper ends ofore pipes 99 which discharge into funnels 39 on the upper end of feedpipes 40 provided with control valves 42.

The preferred ore injectors comprise principally a metallic pipe section49,'the inner end of which is set into the shell of the burner atapproximately the mid-point of the combustion chamber, pipe 49 providingan ore inlet conduit 49. Tube 48 is held in position by circular plate59 and flange 5| bolted to steel shell I 3. The axis of tube 48 isdirected upwardly at a relatively sharp angle, and in the particularembodiment of the apparatus illustrated, this angle may, for example, beabout 60 to the horizontal.

As shown in Fig. 2, valve-controlled pipe 49 projects through the upperside of tube 49. A gas inlet pipe 53 passes through end plate 54,closing the bottom of conduit 49, and terminates at about the dischargeend of pipe 40. Admission of air or other gas to pipe 59 is controlledby valve 55. Plug 56 affords means for cleaning out conduit 49 shouldthe same become clogged up.

The burner may be provided with any suitable number of injectors. In theembodiment of the invention, the two injectors shown in Fig. 1 arediametrically opposed. In some instances, it is desirable to employ saythree or more injectors equally spaced about the circumference of theburner.

Surrounding the burner near the injectors is a bustle 51 for supplyinggas to the injector jets 59, the bustle being connected to a. gas supplythrough pipe 59 having a control valve 56'. Substantially all of the airto support the roasting operation is introduced into the chamber l2through ports 66 and 6| spaced circumferentially about the lower andupper ends of the combustion zone. These ports are connected throughshort pipe sections 69 and 64 with bustles 66 and 61, provided with airinlet connections 69 and 69 having control valves 19 and II. Inlet pipes68 and 69 communicate with an air main 12 one end of which is connectedto the pressure side of a fan or blower 19 providing means to supply airto the burner under positive pressure.

The bottom of the combustion chamber I2 is formed by a hopper shapedbrick-faced hearth l5 terminating in an outlet 11 through which cinderis continuously discharged into a conveyor. As desired, the surface ofhearth I5 may be pitched at an angle greater than or less than the angleof repose of the cinder. In the drawing, hearth 15 is shown as beingpitched at an angle greater than the angle of repose of the cinder, andin this instance the bottom of the hearth may be provided with anupstanding flange "which serves to maintain on the hearth a bed ofcinder indicated at 19. A gas off-take main 9| opens into the combustionchamber at about the mid-point thereof, and preferably slightly belowthe inlet ends of the ore injectors.

Referring to the modified construction shown in Fig. 3, the upper end ofthe burner is constructed substantially as described in connection withFig. 1. The bottom 99 of the combustion chamber 89 pitches downwardlytoward shell II, and is provided at the circumference thereof with oneor more cinder outlets 9|. Bottom 99 has a central opening toaccommodate a hollow rotatable shaft 92 carrying rabble arms 93 eachhaving a plurality of downwardly extending plows 94 for feeding cinderinto openings 9|. Rabble arms 93 are constructed so as to includelongitudinal conduits 95, opening into the hollow interior of shaft 92to permit passage through the arms of cooling air which is dischargedinto the combustion chamber through openings 96. Shaft 92 is supportedin suitable bearings, and is rotated by a gear 91 meshing with a pinion96, driven from a source of power not shown.

Cooling air is introduced through slots 99 into the interior of shaft 92from an air inlet pipe I99, having a control valve I69, terminating in acasing I92 surrounding the lower end of the shaft. Cinder dischargedfrom the combustion chamber through outlet 9| runs through pipe I95 intoa closed chamber I06 having therein a screw conveyor or othersuitable'means for discharging cinder without admitting air to thecombustion chamber.

The modified furnace shown in Fig. 3 is particularly adapted forcarrying out roasting under a slight positive or negative pressure. Itwill be understood the furnace is provided with upper and lower bustles66 and 61, pipe connections 69 and 69, and control valves I0 and H as inthe construction of Fig. 1. When proceding so that at least the lowerportion of chamber 99 operates at negative pressure, air inlet pipe 66may be opened to the atmosphere. On account of the difference intemperature of the gases in the combustion chamber and the outsideatmosphere, the "chimney eflect created inthe combustion chamber 89(Fig. 3) may cause gases in the top of the combustion zone to attempt todivided metal sulfides such as iron pyrites, pyrrhotite, zinc sulfide orarsenopyrite, but for convenience the operation of' the process maypyrites.

A supply of sulfide fines is maintained in the bin 24 by suitableconveyor or elevatormechanism,not shown. Before roasting is begun,combustion chamber I2 is preheated to temperatures above the ignitionpoint of the particular ore to be roasted, as by the use of oilburnersini serted through conveniently located workholes,

not shown. When the desired degree of preheat is obtained in thecombustion chamber, the mo-' tor 22 is started, and rabble arms l9 andsweep 21 may be rotated at a rate of, say, one revolution in twominutes. Fines run continuously out n of the bin 24 onto platform 25,and on each revolution of shaft 23 a regulated quantity of ore is sweptoil the platform to approximately the center of hearth I'l.

During rotation of the rabble arms l9, the sulfide fines are graduallyworked across the heated surface of hearth l1 and into passages 34. The

dry and partly heated ore runs onto sloping screens 35, which removelumps, and thence into ore feed pipes 38 discharging into funnels 33.This preliminary heating serves to dry the fines,

thus facilitating the formation of more satisfactory dispersion of orein the combustion chamber, and at the same time preheats the fines tosome extent. By causing the ore to be roasted to pass over the top ofthe roasting chamber, a largeamount of heat usually lost to theatmosphere is utilized to dry and preheat the fines. By this procedure,the capacity of a burner unit of given size is largely increased.

Valves 42 in pipes 40 are adjusted so that a substantially steady streamof fines runs into feed conduits 49. The next step of the processinvolves injection of the sulfide fines into the combustion chamber,which operation may be satisfactorily effected by use of injectors ofthe type shown in Fig. 2 of thedrawing. These injectors may be placed inthe side walls ofthe furnace, adjacent either the top or bottom but arepreferably located approximately as indicated in the drawing. Air,steam, or other gas, not adversely affecting oxidation of the sulfide,may be employed to charge the fines into the combustion chamber. It ispreferred to employ air for this purpose, and in this case the air maybe admitted to the lower ends of conduits 49 through valve controlledjets 53 at pressures of, for example, about 5 pounds per square inch.

If the ore being roasted is of such nature that extraneous fuel isrequired to maintain proper roasting temperatures, such fuel in anysuitable form may be introduced into the combustion chamber through theore feed mechanism. For example, a combustible gas might be employed toinject the fines.

The angle of the axisof conduit 49 is dependent upon the particular sizeand type of combustion chamber which may be employed. The angle of theaxes of conduits 49, the amount of fines fed into the injectors throughpipes 40,

and the air pressure in jets 53, adjusted by valves anysubstantialquantities of ore particlesand the hot walls of the roastingchamber is prevent-- ed. hus avoiding accumulation of scar on the walls.Referring to Fig. 1, the dottedline H5 in,- dicates the approximate pathof travel of more particle of average size introduced through theinjector on the left lsideof the burner. Two-ormore injectors arepreferably employed. The ore particles move upwardly at an anglesomewhat less than the pitchof injector conduits 43,

the bottom thereof, preferably does notexceed, say, three-quarters ofthe diameterof the roast-i ing chamber. In this manner, contact between5 to an elevation just below crown.l 5, andthen because of comminglingof particles of several individual streams of fines, a relativelyuniform,

dispersion is formed over a major portion of the upper end of thecombustion chamber, and there t after the particles settle in more orless-straight lines, at a rate substantially as induced by grayit).toward the hearthin the bottom of the combustion chamber.

In the preferred proceduraair is supplied to the combustionchamber l2under positive pressure, and substantially the total quantity of airv orother oxidizing gas necessary tosupport the oxidation reaction isintroduced into the combustion chamber through ports 50 and GI at thebottom and at the top of the combustion chamber. Q

Where air is employed'for injecting the fines through conduits 49, as arule not more than about 10% of the total air required for oxidationwould ordinarily be introduced through air jets 53, although largeramounts maybe used if desired. In the preferred operation, where only arelatively small proportion of the total air is fed through theinjectors, it may be considered that substantially ally the air utilizedin .roastingis introduced into the combustion chamber through ports 60and BI at the the furnace. 1

The amount of air introduced through upper,

bottom and at the top of ports 6 I relative to the :amount admittedthrough lower ports 50 may be varied in-accordance with f' the positionof gas off-take 8| For instance, if

the latter is located'so that about 65% -of theroasting is effectedabove the gas ofi-take, the

amount of air admitted through upper ports BI is regulated so as, toprovide in upper portion of the reaction chamber sufiicient air, tosupport about 65% oxidation of the fines, the balance of the air needed.to complete oxidation being admitted through lower ports 60. In thepresent 1 example, it may be considered by suitable adjustment of valvell approximately half of'the air needed for roasting operation may beadmitted into the top of the combustion chamber from pipe 12 throughvairinlet pipe 69, bustle 61 and pipe connections 64 associated with ports6|. By

regulation of valve .10 the balance ofthe air is the inventiondescribed, the diameterand height of the combustion chamber are. about--.equal.- While these proportions are preferred, it is to;

be understood the diameter of the chamber may,

. in some .instances, advantageously exceed, the

height by a substantial amount, and may a so be somewhat less than theheight. Preferably, the

diameter of the chamber is not less, than the height. Since thecombustion chamber preferably has a large diameter, it will be seen thevelocity of the upwardly flowing stream of oxidizing gas is low. Sincethe hot combustion gases are discharged from the reaction chamberthrough pipe II, it will be seen the ore fines are first in-.

troduced into a highly heated section of the combustion chamber, thisfeature serving to aid rapid heating and isnitionof the fines.

At the uppermost point of travel of the fines, a

of similar particles falling under the influence I of gravity. In thepreferred mode ofoperation, the blower 11 is operated so as to feed intothe furnace enough air to supply sufficient oxygen to effectsubstantially complete oxidation of the fines, and also so that thevelocity of the gas stream through the combustion chamber is notsufilcient to interfere with the free gravity fall of the fines. Becauseof the angular pitch of inlets l8, and other above-noted controlconditions, ore particles during the initial movement are not thrownagainst the walls of the roasting chamber, and thus avoid contact of thefines with the hot walls of the combustion chamber when the fines are ina state conducive to scarring. As the downward flow of the fines is insubstantially straight lines or at a high angle, subsequent contact offines with the walls of the combustion chamber is avoided, thus furtherpreventing conditions under which scarring might take place.

Since the gas in the upper end of the combustion chamber comprisessubstantially all air introduced through ports 6|, it will be seen inthe upper end of the roasting zone there is provided an'atmosphere richin oxygen. This procedure insures the presence of relatively largeamounts of oxygen at a point where oxygen is needed to promoteinitiation of the roasting of the ore particles. By the time downwardmovement of the ore particles begins, roasting is well under way, andduring the remainder of the fall of the ore particles through thesection of the combustion chamber above the gas ofi'take 0|, the oreparticles and the gas stream flow co-current. At about the verticalmid-point of the combustion chamber II, the sulfur content of the orehasbeen materially reduced, and further oxidation of the fines tends toproceed less readily. The partially roasted particles then drop into theupper end of the lower section of the roasting zone, and duringcontinued fall, the ore particles pass countercurrent through a streamof oxidizing gas drawn in through ports 60 and fiowing upwardly towardgas oiftake 8|. In the lower half of the combustion chamber the oreparticles thus pass through an atmosphere increasingly rich in oxygenand containing decreasing smaller amounts of sulfur dioxide. Thisprocedure provides for the presence of successively larger amounts ofoxygen as oxidation of the particles becomes more difficult and hastensremoval of sulfur dioxide from the atmosphere surrounding the individualparticles. Just above and at the inlet ports 60 the ore particles dropinto and through an atmosphere which is substantially all air, acondition which greatly facilitates completion of oxidation of thecinder.

The iron oxide cinder falling on hopper-shaped hearth I is finelydivided, free-flowing, and runs through opening 11 into a suitableconveyor.-

The hot cinder collecting on hearth l5 radiates heat upwardly thusaiding in maintaining favorable temperature conditions in the roastingchamber. It will be understood the roasting operation is conducted sothat in the combustion chamber there is a very slight negative pressureat opening ll, whatever small amount of'air drawn in being utilized foroxidation;

In the furnace of Fig. 1, it may be considered the pitch of the surfaceof hearth II is greater than the angle of repose of the cinder. Withthis construction, the hot cinder at temperatures of about 1400-1600 F;collecting on the hearth radiates heat upwardly thus aiding inmaintaining favorable temperature conditions. in the roasting chamber.

When carried out in apparatus such as shown in Fig. 3, the operation ofthe process is substantially as described. In this furnace, the majorportion of gas movement may be efi'ected by suction induced by a fan ingas main II or, when air is supplied under suitable positive pressurethrough ports, ii and 60, the furnace may be operated under slightpositive pressure.

. Although the burner may be operated so that all portions of thecombustion chamber are under negative pressure, 1. e. the air beingdrawnin through pipes 68 and is opened .to the atmosphere, for the reasonsindicated above, it is usually more desirable to feed air into the topof the combustion chamber under slight pressure. It will be understoodthe amount of air introduced into the burner is regulated by adjustmentof valves such as 10 and II.

In Fig. 3, the cinder falls onto the bottom 00 and the lower ends ofplows 94 and rabble arms 93 are so adjusted as to facilitate themaintenance on the bottom of the combustion chamber of a bed of cinder.of appreciable thickness, say 2 to 4 inches. As the area of the cinderbed is relatively extensive and the temperature of the cinder may bearound say 1400-1600 F., large quantities of heat are radiated from thecinder bed upwardly into the combustion zone. The burner may be operatedunder conditions such that the velocity of the gas stream flowingthrough the combustion chamber is substantially the same as in theoperation of the apparatus of Fig. l. Rabble'arms 93 are rotated at asuitable rate, and cinder is ultimately discharged from the bottom ofthe combustion chamber through one or more air-locked outlets I05. If itshould be desired to introduce preheated air through the ports at thebottom and top of the furnace of Fig. 3, such air may be preheated byheat transfer from hot cinder after discharge of the latter from thecombustion chamber, or by heat transfer from hot combustion gasesdischarged through pipe Air used in the furnace of Fig. 1 may bepreheated similarly if so desired.

The sulfur dioxide gases produced and withdrawn through line 8! may beused, for example;

in the manufacture of sulfuric acid. The sulfur dioxide content of theburner gas may be regusection thereof, said conduits being inclined athigher than the inlet ends of the ore feeding conduits, means forworking ore over the surface of the top, means for feeding the ore intosaid conduits, gas jets in said conduits for charging ore into thecombustion chamber, a plurality of gas inlet ports spaced about theperiphery adjacent the top and base of the shell, means for introducinggas into the combustion chamber through said ports, means fordischarging cinder from the bottom of the combustion chamber, and a gasoutlet for the combustion chamber adjacent the midsection thereof.

2. Apparatus for roasting finely divided metal sulfide fines'comprisinga shell forming a vertically disposed combustion chamber, a plurality offines feeding conduits spaced about the periphery of the shell adjacentthe mid-section thereof, said conduits being inclined at a relati velyhigh angle to the horizontal and opening into the combustion chamber atpoints higher than the inlet ends of the fines feeding conduits, meansfor feeding the fines into said conduit, gas jets in said conduits forcharging fines into the combustion chamber, a plurality of gas inletports spaced about the periphery adjacent the top and base of the shell,means for introducing gas into the combustion chamber through saidports, means for discharging cinder from the bottom of the combustionchamber, and a gas outlet for the combustion chamber adjacent themidsection thereof.

3. The method of roasting finely divided sulfides to produce sulfurdioxide which comprises introducing sulfide fines into a substantiallyunobstructed combustion zone in an upward direction having a substantialhorizontal component to form a dispersion of sulfide fines adjacent the,top of the combustion zone, passing the fines downwardly in gaseoussuspension through the upperportion of the combustion zone co-currentwith a stream of oxidizing gas and the resulting gaseous combustionproducts, and then through the lower portion of the combustion zonecountercurrent to a stream of oxidizing gas, the total amount ofoxidizing gas present being sufilcient to effect substantially completeoxidation of the fines, discharging cinder from the bottom-of thecombustion zone, and withdrawing sulfur dioxide from an intermediateportion thereof.

4. The method of roasting finely divided sulfides to produce sulfurdioxide which comprises radially introducing sulfide fines upwardly intoa substantially unobstructed combustion zone heated to temperaturesabove the ignition point of the fines, forming a dispersion of the finesadjacent the top of the combustion zone, passing the fines downwardly ingaseous suspension through the upper portion of the combustion zonecocurrent with a stream of oxidizing gas and then through the lowerportion of the combustion zone countercurrent to a stream of oxidizinggas, the initial oxygen concentrations of said streams beingsubstantially equal and the total amount of oxygen contained in saidstreams being sufllcient to efiect substantially complete oxidation ofthe fines, discharging cinder from the bottom of the combustion zone,and withdrawing sulfur dioxide gas at a point adjacent the verticalmid-point of the combustion zone.

and adjacent the bottom of the combustion chamber for introducingoxidizing gas into the combustion chamber, a gas outlet for thecombustion chamber adjacent the mid-section thereof, and a cinder outletat the bottom of the combustion chamber. Y

6. Apparatus for roasting finely divided sulfide fines comprising ashell forming a combustion chamber, means including an upwardlydirected, radially disposed injector passing through a side wall oftheshell for forming a dispersion of fines in the top of the combustionchamber, means for feeding fines to said injector, means including inletports adjacent the top and adjacent the bottom of the combustion chamberfor introducing oxidizing gas into the combustion chamber, a gas outletfor the combustion chamber adjacent the mid-section thereof, and acinder outlet at the bottom of the combustion chamber.

7. The method of roasting finely divided sulfides which comprisesintroducing sulfide finesbetween the ends and in an initial directiontoward one end of a substantially unobstructed combustion zone to form adispersion of sulfide fines adjacent said end of the combustion zone,maintaining said combustion zone at temperatures above the ignitionpoint of the fines, introducing oxidizing gas into said end and into theopposite end of the combustion zone, the total amount of oxidizing gasbeing such as to supply to the combustion zone sufilcient oxygen tosupport oxidation of the fines, passing the fines in gaseous suspensiontoward said opposite end through the combustion zone first through aportion of'the oxidizing gas and having in said portion an metalatmosphere decreasingly rich in oxygen and then 7 through a secondportion of the combustion zone countercurrent to a stream of oxidizinggas and having in said second portion an atmosphere increasingly rich inoxygen, discharging cinder from said opposite end of the combustionzone, and withdrawing sulfur dioxide gas from a point between saidportions.

8. The method of roasting finely divided sulfides to produce sulfurdioxide which comprises radially introducing sulfide fines into asubstantially unobstructed combustion zone in an upward direction havinga substantial horizontal component to form a dispersion of sulfide finesadjacent the top of the combustion zone, passing the fines downwardly ingaseous suspension through the upper portion of the combustion zoneco-current with a stream of oxidizing gas and the resulting gaseous comustion products, and then through the lower port on of the combustionzone countercurrent to ,a stream of oxidizing gas, the total amount ofoxidizing gas present being sufilcient to effect substantially complete.oxidation of the fines, discharging cinder from the bottom of thecombustion zone, and withdrawing sulfur dioxide from an intermediateportion thereof.

culmina- FORBES smear.

